The present invention relates to the development of thermally responsive films and has its most important application to fine grain, high-resolution, organosilver photorecording mediums, commonly called dry silver films. Such films are disclosed in U.S. Pat. No. 3,152,903 and 3,152,904 assigned to the Minnesota Mining and Manufacturing Company. The manufacturers of these films commonly publish for each film a curve which shows the recommended development times at various assumed constant temperatures applied to the films. While satisfactory film development also occurs where the film temperature deviates a small amount, like plus or minus 1/2.degree. C. from the assumed constant development temperature, such is not generally the case for temperature deviations of the order of magnitude of several degrees centigrade from the desired control temperature.
Heat for developing these films is most conveniently applied to the films by a heat-applying metal plunger having a current carrying heating element therein. The plunger is moved into engagement with the film for a desired developing period. It was unexpectedly found that even when the current in the heating element was constant, the temperature of the film engaging end face of the plunger varied quite substantially even over a period of time as little as several seconds during which the plunger is normally applied to the film. These fluctuations, which are substantially beyond 1/2.degree. C., are believed to be caused by moving air currents adjacent the plunger and film. Temperature fluctuations are also caused by variations in the line voltage supplying current to the heating element in the plunger and the humidity and room temperature conditions. The effect of the variables involved on the plunger end face temperature is especially pronounced when the mass of the plunger is relatively small. While a temperature controller can be provided for controlling the current flow in the heating element to maintain roughly a given temperature at the end face of the plunger, such a controller cannot effectively stabilize the plunger end face temperatures under carrying air current or heater current conditions because there is a time lag between an increase or decrease in current fed to the heating element and the effect such current change has on the temperature at the end face of the plunger. It is, therefore, practically impossible to keep the temperature of the end face within plus or minus 1/2.degree. C. in environments where air current or voltage fluctuations occur. Furthermore, a controller which can control temperature within such narrow limits even under slowly varying conditions is a costly piece of equipment.
Accordingly, one of the objects of the invention is to provide a unique film developing system for controlling the time a heat source is applied to a thermally responsive film, like the dry silver films referred to, so that optimum developing times are achieved automatically under heat source temperature fluctuating conditions, like variations in the movement of air currents, and the fluctuation of line voltage, and room temperature and moisture conditions. Another object of the invention is to provide a film developing system as described where the equipment involved is reliable and relatively inexpensive to manufacture.